The intestine is exposed to bacterial flora, dietary antigens and potential pathogens. To prevent chronic inflammation, various cell populations keep the mucosal immune response in check. Interleukin 10 (IL-10) is known as a cytokine with anti-inflammatory properties. IL-10 and IL-10 receptor (IL-10R) deficient mice develop spontaneous colitis in the presence of a normal intestinal microflora. Recent reports identified IL-10 as a susceptible locus for the development of inflammatory bowel diseases (IBD). Moreover IL-10R gene variants were found in some patients with early-onset colitis. Many studies identify CD4+ T cell-derived IL-10 as a key mediator of intestinal immune homeostasis, however, we have found IL-10 derived from intestinal F4/80+ CD11b+ CD11cint macrophages acts directly on regulatory T cells (Treg) and maintains their suppressive function in a mouse model of colitis. This suggests that IL-10 from macrophages rather than T lymphocytes also could be critical. Therefore the experiments in this application will focus on the cell type in the intestine that produces IL-10. We will investigate its phenotype, it's possible mode of regulation, and its broad effects on mucosal immunity including regulatory T cell function, using genetic technologies, cellular immunology methods, bioinformatics technologies and in vivo disease models. In Aim 1, we will examine how IL-10 producing macrophages modulate immune responses using mice with cell type-specific targeted deletion of the Il10 gene. We will fully characterize the phenotype and function of the large intestinal IL-10 producing macrophages compared with IL-10 non-producing cells in the regulation of mucosal homeostasis (Aim 2). We believe that our experimental system has relevance to the possible development of macrophage-based immune therapy in the future. In summary, the proposed experiments build on our novel preliminary findings in order to achieve a deep understanding of the pathway leading to IL-10 production by intestinal macrophages, and the means by which gut homeostasis is regulated. PUBLIC HEALTH RELEVANCE: Interleukin 10 (IL-10) is a molecule that decreases inflammation. It protects against inflammation in the intestine of mice and may also play the same role in humans with inflammatory bowel disease (IBD). We will try to understand what cells produce this molecule and how this results in a decrease in inflammation. If we can understand how this works in mice we might be able to better understand, and eventually treat, IBD in humans. !